The present invention relates to a method of regenerating a washing solution, which contains Iron (II)-chelate complex of ethylenediaminotetraacetic acid (EDTA) as a complex-forming substance, and is used for a simultaneous washing of nitric oxide and sulfur dioxide out of exhaust gases.
Due to a more and more intensive environment contamination various efforts have been made to avoid emission of sulfur dioxide and nitric oxide. This is particularly important for large heating installations, such as power plants. It is, for example provided that for all new installations of power plants, the output of which exceeds 300 MW, the amount of NOx-output should be limited to 200 mg/m.sup.3. However, research conducted up to now has failed to offer a method which would enable a complete removal of the above noxious materials from exhaust gas.
It has been already suggested to carry out the washing of exhaust gas with a washing solution which contains, as a complex-forming substance, Iron (II)-chelate compound of ethylenediaminotetraacetic acid (EDTA). Here sulfur dioxide in the watery agent of the washing solution is bound under the formation of sulfurous acids or salts according to the following reaction: EQU SO.sub.2 +H.sub.2 O.revreaction.H.sub.2 SO.sub.3
The washing solution according to p.sup.H -value results in the formation of hydrogensulfite (p.sup.H 23 5) or sulfate (p.sub.H .gtoreq.9). Thus during the washing of the exhaust gas the gas is treated in weak acid agents (p.sup.H 5-7). At the same time nitric oxide is bound by Iron (II)-chelate compound of ethyleneaminotetraacetic acid (EDTA) contained in the washing solution according to the following reaction: EQU Fe.sup.2+ -EDTA+NO.revreaction.Fe.sup.2+ -EDTA.times.NO
This means that the advantage of such a method resides in that a simultaneous washing of nitric oxide and sulfur dioxide out of the exhaust gas is possible in a single apparatus. In connection with this washing, NO-molecules deposited on Iron (II)-chelate complex are transmitted by ions of sulfite and hydrogen sulfite, contained in the washing solution, after the separation from the complex, into sulfur and nitrogen compounds whereas simultaneously the Iron (II)-chelate complex is regenerated or efficiently renewed.
Research has shown that even small amounts in other nitrogen - sulfur compounds substantially result in imidodisulfonate NH (SO.sub.3).sub.2.sup.2-. The following reaction, for example, takes place: EQU 2NO+5HSO.sub.3.sup.- .fwdarw.2NH(SO.sub.3).sub.2.sup.2- +H.sub.2 SO.sub.4 -.sub.+H.sbsb.2.sub.O
If p.sup.H value of the washing solution is &gt;7 NO.sub.2 is formed, which escapes in gaseous state. Therefore, as has been noted above, it is important to carry out an exhaust gas washing in a weak acid agent and to thereby avoid the N.sub.2 O-formation.
Inasmuch as the washing solution normally flows through a circuit it becomes in time enriched with imidodisulfonate which can specifically lead to damaging of the washing process due to the separation of crystallized imidodisulfonate. It is therefore required to undertake suitable measures, which would ensure that this undesired enrichment of imidodisulfonate in the washing solution be avoided and resulting disturbances be prevented.